We report the experimental evidence of the ac Josephson effect in a transition edge sensor (TES) operating in a frequency domain multiplexer and biased by ac voltage at MHz frequencies. The effect is observed by measuring the non-linear impedance of the sensor. The TES is treated as a weakly-linked superconducting system and within the resistively shunted junction model framework. We provide a full theoretical explanation of the results by finding the analytic solution of the non-inertial Langevian equation of the system and calculating the non-linear response of the detector to a large ac bias current in the presence of noise.Superconducting transition-edge sensors (TESs) are highly sensitive thermometers widely used as radiation detectors over an energy range from near infrared to gamma rays. In particular we are developing TESbased detectors for the infrared SAFARI/SPICA 1 and the X-ray XIFU/Athena 2 instruments. TESs are in most cases low impedance devices that operate in the voltage bias regime while the current is generally read-out by a SQUID current amplifier. Both a constant or an alternating bias voltage can be used 3,4 . In the latter case changes of the TES resistance induced by the thermal signal modulate the amplitude of the ac bias current. The small signal detector response is modelled in great details both under dc and ac bias 5,6 . Those models however do not fully explain all the physical phenomena recently observed in TESs. It has been recently demonstrated that TES-based devices behave as weak-links due to longitudinally induced superconductivity from the leads via the proximity effect 7 and a detailed experimental investigation of the weak-link effects in dc biased x-ray microcalorimeters has been reported 8 . Evidence of weaklink effects in ac biased TES microcalorimeters has been given 9 , but an adequate experimental and theoretical investigation is still missing. We previously proposed a theoretical framework 10 based on the resistively shunted junction model (RSJ) that can be used to describe the resistive state of a TES under dc bias. In this letter, we extend the model to calculate the stationary non-linear response of a TES to a large ac bias current in the presence of noise and we compare it to the experimental data obtained with a TES-based bolometer. We report a clear signature of the ac-Josephson effect in the TES biased at MHz frequencies.The general equation for the Frequency Domain Multiplexing (FDM) electrical circuit, simplified for a single a) Electronic mail: l.gottardi@sron.nl resonator is 6(1) where V (t) is the total voltage across the TES, L and C are respectively the inductance and the capacitance of the bias circuit, r s is the total stray resistance in the circuit and Z T ES is the TES impedance, which depends on temperature T and current I(t). As previously reported 8,11 , the superconducting leads proximitize the TES bilayer film over a distance defined by the coherence length ξ. As a result, the superconducting order parameter |Ψ| is spatially dependent over the ...

We present new results on the development of larger arrays (presently 72 channels, goal of multiple units of 160 channels) of superconducting LC filters. The a-Si:H based resonators show a quality factor above 10.000. The latest design utilizes oppositely wound planar coil pairs which enable close packing with low magnetic cross talk. We present results on the obtained center frequency distribution within the range of 1 to 3 MHz.

Josephson Effects in Frequency-Domain Multiplexed TES Microcalorimeters and Bolometers Gottardi, L.; Smith, S. J.; Kozorezov, A.; Akamatsu, H.; Bruijn, M. P.; Chervenak, J. A.; Gao, J. R.; Jackson, B. D.; Ridder, M.; More AuthorsAbstract Frequency-division multiplexing is the baseline read-out system for large arrays of superconducting transition-edge sensors (TES's) under development for the X-ray and infrared instruments like X-IFU (Athena) and SAFARI, respectively. In this multiplexing scheme, the sensors are ac-biased at different frequencies from 1 to 5 MHz and operate as amplitude modulators. Weak superconductivity is responsible for the complex TES resistive transition, experimentally explored in great detail so far, both with dc-and ac-biased read-out schemes. In this paper, we will review the current status of our understanding of the physics of the TES's and their interaction with the ac bias circuit. In particular, we will compare the behaviour of the TES nonlinear impedance, across the superconducting transition, for several detector families, namely: high-normal-resistance TiAu TES bolometers, low-normal-resistance MoAu TES microcalorimeters and high-normal-resistance TiAu TES microcalorimeters.

Athena is designed to implement the Hot and Energetic Universe science theme selected by the European Space Agency for the second large mission of its Cosmic Vision program. The Athena science payload consists of a large aperture high angular resolution X-ray optics (2 m2 at 1 keV) and twelve meters away, two interchangeable focal plane instruments: the X-ray Integral Field Unit (X-IFU) and the Wide Field Imager. The X-IFU is a cryogenic X-ray spectrometer, based on a large array of Transition Edge Sensors (TES), offering 2:5 eV spectral resolution, with ~5" pixels, over a field of view of 50 in diameter. In this paper, we present the X-IFU detector and readout electronics principles, some elements of the current design for the focal plane assembly and the cooling chain. We describe the current performance estimates, in terms of spectral resolution, effective area, particle background rejection and count rate capability. Finally, we emphasize on the technology developments necessary to meet the demanding requirements of the X-IFU, both for the sensor, readout electronics and cooling chain

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